This invention relates to precision navigation devices, and particularly, to devices which compare a reference image with a scene of terrain being overflown by manned or unmanned flying bodies, e.g. aircraft.
Precision navigation devices with image comparison are known. In the underlying process used in such devices, the so-called comparison navigation process, comparison is made between the terrain scanned at the time it is flown over and a previously recorded reference image.
U.S. Pat. No. 2,526,682 discloses a flight indicator in which radar installed in the aircraft surveys the overflown terrain. The terrain is displayed on a cathode-ray tube. At the same time, a projector projects a previously taken film of the terrain on a translucent projection screen on whose other side the projection screen of the cathode-ray tube is arranged. An optical lens projects the image of the cathode-ray tube on the projection screen in the correct position so that an observor can see both images on the projection screen. Conclusions can be reached about the altitude of the aircraft, and its speed above ground, as well as its position, from the deviation between the images.
U.S. Pat. No. 3,103,008 discloses a similar device. The projection screen displays an actual positive of the overflown terrain. The image can be made by radar, and infrared conversion, or any photographic positive. A previously taken negative reference image of the terrain is arranged parallel to the positive. After this, an optical lens and a following photocell are arranged to scan both images synchronously. Servomotors adjust the images on the basis of the scanning results until a maximum correlation is achieved. The deviations permit determination of the position of the aircraft in relation to the reference image.
Another apparatus and method of this type is shown in "Aviation Week", June 23, 1958, page 53. Here, actual scenes of the terrain are scanned by radar and compared with photographic reference images. These are carried along and constitute an atlas of images on the film.
U.S. Pat. No. 3,163,098 discloses a device in which a camera continuously photographs the overflown terrain. The terrain lying ahead of the flight is sensed and compared with a stored image of the same terrain after it has been overflown. The comparison is combined with other factors to influence control of the aircraft.
"Aviation Week & Space Technology", Feb. 25, 1974, page 50, ff, discloses the Tercom process. In the latter, barometric measuring devices and radio altimeters produce altitude profiles during specific time intervals of a flight over characteristic terrain. The one-dimensional differential profile between the barometric altitude and altitude above ground is compared with a two-dimensional reference profile. Here, the measured altitude profile is adjusted until the best correlation is achieved, so that the exact position of the aircraft results.
Another procedure is disclosed in "Aviation Week & Space Technology", Apr. 5, 1976, pages 39 etc. Here, the chharacteristic radiation of different materials in the microwave range is utilized. At given intervals, a microwave radiometer measures the characteristic radiation temperatures of the overflown terrain at given intervals. Maps which show only steps in the radiation temperature are used as a reference. These are produced from multispectral satellite photographs taken at corresponding seasons of the year.
An object of the present invention is to improve precision navigation devices.
Another object of the invention is to produce a device of the type described above which makes possible a continuous comparison of the overflown terrain with a minimum number of devices, and in which the data to be handled are limited to the necessary minimum.